Surface hardening of aluminum alloys

ABSTRACT

This invention provides aluminum alloys which have a Vickers surface hardness of about 250 kg/mm2 to 1,400 kg/mm2. This invention also provides a process for preparing such aluminum alloys by exposing said aluminum to a source of cyanide anion at a temperature of about 450*C. to 550*C.

United States Patent Weinbaum [45] Oct. 28, 1975 SURFACE HARDENING OFALUMINUM [56] References Cited ALLOYS UNITED STATES PATENTS [75]Inventor: Richard Martin Otto Weinbaum, 2,413,929 1 1947 Simpson 148/20S210 Paulo, Brazil 3,268,372 8/1966 Brotherton et a]. 148/20 [73]Assignee: Metal Leve S.A., Sao Paulo, Brazil Primary Examiner Ralph SKendall [22] Filed: Apr. 25, 1973 Assistant ExaminerCharles R. Wolfe,Jr.

Attorney, Agent, or FirmLadas, Parry, Von Gehr, [21] Appl 354619Goldsmith & Deschamps Related US. Application Data [63]Continuation-in-part of Ser. No. 250,126, May 4, [57] ABSTRACT 1972abandmed' This invention provides aluminum alloys which have a Vickerssurface hardness of about 250 kg/mm to [52] US. Cl. l48/6.11; 148/627;148/20; 400 2 i invention also provides a process 51 I Cl 2 1418/28148/315 for preparing such aluminum alloys by exposing said 1 lrt. l ito a Source of Cyanide anion at a p Fleld Of Search 1, A, 20, ture ofabout 450C to 5500C 7 Claims, 19 Drawing Figures US. Patent FIG. l

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Oct. 28, 1975 Sheet 1 of 8 U.S. Patent Oct. 28, 1975 Sheet 2 of 83,915,758

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US. Patent Oct. 28, 1975 Sheet 4 of8 3,915,758

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if) R SURFACE HARDENING OF ALUMINUMALLOYS This application is acontinuation-in-part of my prior copending application Ser. No. 250126filed May 4, 1972, now abandoned, and the benefit of the filing date ofmy prior application is claimed.

BACKGROUND OF THE INVENTION a. Field of the Invention It is obvious thatthe hardening of the surface of aluminum alloys is an extremelydesirable phenomenon. Thus, the very best possible use of a lightweightmetal such as aluminum could result if the hardness of aluminum could beincreased so that it could be utilized in areas where heavier but hardermetals (e.g.,-steel) are now used. To overcome thedrawback resultingfrom the relative softness of aluminum it has been the practice toreplace aluminum portions with other metal portions at that point wherehardness or surfaceresistance is critical. Thus, the points of severewear and abrasion in machine parts are made-from metals other thanaluminum, whereas, the remaining portions of many machines are made fromaluminum.

By having to substitute heavier metals in certain portions of machinerywhich would otherwise be made from aluminum at least two drawbacksresults-The first is that inexpensive single piecesof machinerycomponents cannot be made and expensive piecing together of machineparts have to be carried out. Thesecond drawback is that the morealuminum which has to be replaced by heavier metals the heavier themachinery is. This latter drawback is particularly severe in theaircraft industry.

b. Description of the Prior Art There have been studies of the treatmentof aluminum, but none have really been directed to the problem ofincreasing the surface hardness of aluminum alloys.

On Mar. 18, 1935 Professor M. H. LeChatelier, published a paper in theAcademie des Sciences authored by Paul Laffitte and Pierre Grandadamentitled The Nituration of Several Metals (pages 1,039 to 1,041). Asecond paper was published Aug. 1, 1936 by M. P. Laffitte, M. M. E.Elchardus and P. Grandadam in the Revue de lindustn'e Minerale No. 375,page 861, entitled Research on the Nituration of Magnesium and Aluminum.A third paper was published in July and August 1935 in the Ann. deChimie l lth series, t.4pages 1 18 to 123 by Pierre Grandadam, entitledExperiments on the Direct Oxidation of Platinum and the Nituration ofSeveral Metals (Cu, Al, Mg, Zn, Fe, Ni, Ti). In all three of thesepublications experiments were described as the effect of nitrogen andammonia at high temperatures on pure aluminum wire. No mention at all ismade in these publications with respect to cast aluminum alloys or tocast and mechanically worked aluminum alloys or to the use of sources ofcyanide ions as I employ in my invention.

A fourth publication is one published by the German Company Degussa inDecember 1970 and authored by Bruno Finnein entitled Wear ResistantSurfaces by Treating Titanium and Titanium Alloys in Salt Baths. In thispublication titanium or its alloys are treated with salt bathscontaining cyanide ions at a temperature of 800C. Of course, experimentswith titanium bear no relationship with resect to aluminum and, inaddition, the use of temperatures in the range of 800C. could notpossibly be used on aluminum alloys. Two other 2 reasons explain why theDegussa article bears no teaching whatsoever with regard to alluminumalloys. This Degussa article does not inform one of the compositions ofthe bath butmerely states that carbon and nitrogen form mixed crystalswith titanium, which seems entirely doubtful to me. My opinion is thatvanadium carbides and aluminum nitrides may be formed and thesecompounds may be responsible for the formation of a deep layer. Degussa,however, stages that the composition of the layer is unknown. Further inour case, I know that silicon, magnesium and nickel are necessary. Thedifference between treatment of titanium and treatment of aluminumappears to mainly consist in that the hard layer, in the case ofaluminum, is mostly produced by alloy constituents of the interior ofthe Al alloy, such as silicon, whereas the hard layer of the titaniumalloy is formed by direct reaction between the bath and the surface ofthe Ti alloy and there appears to be no migration of constituents fromthe interior to the surface.

SUMMARY OF THE INVENTION In accordance with this invention there isprovided a cast alloy of aluminum or a cast and mechanically workedalloy of aluminum containing substantial quantities of silicon, copper,nickel and magnesium and, optionally, small quantities of iron, titaniummanganese, zinc and chromium, characterized by Vickers surface hardnessof about 250 kg/mm to about 1,400 kglmm depending on the composition ofthe alloy and its treatment. This is to be contrasted with a Vickerssurface hardness for the untreated aluminum alloy in the range of kg/mmThis increased surface hardness of said aluminum alloys is effected bytreating said aluminum alloy with a source of cyanide ion at atemperature of 450 to 550C.

The alloying elements content in the aluminum alloys which can betreated by the process of this invention are those having thecomposition:

Si 0.2 to 30.0 Cu 0.2 to 5.6

Ni 0.2 to 6.0

Mg 0.2 to 5.0

Fe 0.0 to 1.0

Ti 0.0 to 0.2

Mn 0.0 to 1.5

Zn 0.0 to 2.0

Cr 0.0 to 0.7

The usual source of cyanide ion is sodium cyanide. Other cyanide sourcescan be used, for example, potassium cyanide.

The temperature at which the aluminum alloy is kept in contact with thesource of cyanide is preferably at about 450 to 550C; At thistemperature a time of exposure of the aluminum alloy to the cyanidesource is preferably for a period of time of preferably from about 8 toabout 12 hours.

In the heating time there is about one half hour incubation. Timeslonger than 20 hours do. not seem to be practical due to the parabolicrelationship of time X depth of the layer. Shorter times in the order ofabout 4 hours treatment showed good results and a smaller surface layerdepth resulted but it had the same hardness as with longer treatments.Furthermore, the zone behind this layer is not completely depleted ofsilicon crystals. This fact is very important from the point of view ofthe hardness gradient from the surface to the 3 core and of theanchorage of the surface layer in the matrix.

Studies of the treated alloys were conducted. A review of themicrographs of the treated alloys indicated the growth of an oxide layersimultaneous with the development of the hard particle layer.

X-Ray diffraction studies were conducted on the sur face of treatedalloys 124 and 138. The results showed that the surface of the treatedalloys were coated with a spinel type oxide having a lattice parameterof 8.07A iO.2A. Two compounds, NiAl O (8.05A) and MgAl- (8.08A) arepotential matches for the data, but electron microprobe data showed thatthe oxide contains Mg. Thus, MgAl O is the major constituent in theoxide layer. This layer contains numerous metallic particles which havebeen identified as Al Ni by X-ray diffraction which was consistent withmicroprobe data showing that these particles contain Ni. Also, thediffraction pattern and microprobe data show elemental Si to be presentin the oxide layer. There was no indication of the oxidation of eitherSi or Ni.

In practice the cast aluminum alloy or cast and mechanically workedaluminum alloy was immersed partially or completely in the salt bath forthe period of time and, after the exposure was completed, it wasconvenient to rinse the test piece with water. Hardness tests were thenrun on the treated aluminum surfaces utilizing the Vickers hardness testE: 92-55 as described in the 1955 Book of ASTM Standards, Part 1,Ferrous Metals, published by the American Society for Testing Materials,l9 16 Race Street, Philadelphia, Pa., at pages 1694 to 1699, hereafterreferred to as Vickers Hardness. The magnitude of the test load used was35 grams.

In preparing the salt bath, it is convenient to mix with the cyanidesalt fluxes or other auxiliary agents which enable the salt bath to bekept conveniently at the 450 to 550C. temperature. Examples of suchagents are the use of boron oxide or the use of a mixture of sodiumhydroxide and sodium carbonate. These auxiliary agents do not enter intothe reaction and merely permit the cyanide ion to be brought into aconvenient form at the 450 to 550C. range in contact with the aluminumaly.

THEORY OF THE PROCESS While I do not want the interpretation of theclaims for my invention to be restricted in any manner by any theory itseems that the phenomenon of surface hardness is connected with themigration of silicon from the interior to the surface, leaving behindthe surface a zone of lower silicon content. The mobility of siliconatoms increases with the silicon content to a maximum of approximately18 (e.g., alloy 138). With a further increase of silicon content thismobility decreases as for example with alloy 244 which has a siliconcontent of about 24. In addition to the silicon migration, the surfacehard layer appears to comprise some Al Ni. Although the increase ofhardness appears to be due mainly to the increase in silicon content inthe hard surface layer, the hardness of alloy Y must be considered froma different point of view. The silicon content of the Y alloy is only0.5 percent and con sequently'there is no essential migration ofsilicon. The hardness of the surface layer of the Y alloy can beattributed mostly to the formation of MgAl O which presumably occurs asa result of oxidation during the salt bath treatment.

Obviously a combination of concentrating silicon and/or Al Ni on thesurface, as well as, the formation of MgAl O can occur to produce thesurface hardness of the aluminum alloy as a result of the treatment ofthis invention. In addition, other undiscovered effects may occur.

That there are several different particles in the hard layer is really avery favorable aspect as far as wear resistance is concerned. Differenthardness is parallel to different elasticity, which reduces the dangerof removing brittle particles.

The oxide layer on the extreme outside has a different composition,mostly MgAl O as compared with the hard particle layer, which consistschiefly of Si crystallites. An exposure time of merely 4 hours produceslayers with less thickness but practically the same hardness. It shouldbe pointed out that hardness is a property of the crystallites 'formedwhile thickness of the hard surface layer depends on exposure time. Wearresistance, on the other hand, depends on both these factors: increasedhardness usually increases wear resistance, and greater thicknessimplies in longer abrasion time of the layer.

The oxide layer consists mostly of MgAl O The oxide layer may contributeto the overall hardness; but its hardness is much below that of the Silayer. The hard layer consists of Si and some Al Ni. Also here Al Ni hasa lower hardness than Si particles.

Even a short exposure time of 4 hours produces a hard layer of reducedthickness in alloys 124, 138 and 244.

Time of exposure and temperature have a certain influence on the hardlayer thickness. I found a parabolic relation for dependence ofthickness on exposure time:

p=K V? DESCRIPTION OF THE FIGURES FIGS. 1 through 12 and 14 and 16 arephotomicrographs of the alloys treated in accordance with thisinvention.

FIGS. 13 and 15 are photomicrographs of untreated alloys.

The data with regard to these Figures are as follows:

Unetched x Alloy l24 Treatment: NaCN 4% B 0 Temperature 530C; Time 4hours Thickness of the hard layer 13 microns.

'fFIG. 2

Unetched 150x Alloy 124 Treatment: NaCN 4% B 0 Temperature 530C; Time 8hours Thickness of the hard layer 18 microns.

FIG. 3

Unetched 150x Alloy l24 -continued Treatment: NaCN 4% B Temperature530C; Time 12 hours Thickness of the hard layer 20 microns. FIG. 4

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C; .Time 4hours Thickness of the hard layer 8 microns.

FIG. 5

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C; Time 8hours Thickness of the hard layer 15 microns.

FIG. 6

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C.; Time12 hours Thickness of the hard layer 17 microns.

FIG. 7

Alloy 244 Unetched 150x Treatment: NaCN 4% E 0 Temperature 530C; Time 4hours Thickness of the hard layer 12 microns.

FIG. 8

Unetched 150 x Alloy 244 Treatment: NaCN 4% H 0 Temperature 530C; Time 8hours Thickness of the hard layer 14 microns.

FIG. 9

Unetched 150x Alloy 244 Treatment: NaCN 4% B 0 Temperature 530C; Time 12hours Thickness of the hard layer 25 microns.

FIG. 10

Unetched 150x. Alloy 244 Treatment: NaCN 4% B 0 Temperature 520C; Time12 hours.

Thickness of the hard layer 10 microns.

FIG. 11

Unetched 150x Alloy 124 Treatment: NaCN 4% B 0 Temperature 520C; Time 12hours The start of migration of Silicon to the surface of the test piececan be observed.

FIG. 12

Unetched 590x Alloy 124 Treatment: NaCN 4% B 0 Temperature 530C.; Time12 hours Indentation of micro-hardness (35 grams) Hardness of matrix 104kg/mm Hardness of the hard layer 1200 kg/mm FIG. 13

Alloy 124 X120 not etched Condition: as cast FIG. 14

Alloy 124 X120 not etched Conditions of treatment: Bath =-90% NaCN 3%NaOH 1% Na CO 6% H O; Temperature 540C; Time 13 hours Thickness of thehard layer 15 microns FIG. .15

Alloy 138 X120 not etched Condition: as cast FIG. 16

Alloy 138 X120 not etched Bath 90% NaCN 3% NaOH 1% Na CO 6% H O;Temperature 540C; Time 9 hours Thickness of the hard layer 6 microns.

Conditions of treatment:

FIGS. 17 and 18 show the relation of the layer thickness to the exposuretime.

FIG. 19 shows the results of wear tests made with Alloy 124.

Alloy 124 was treated for four hours at 540C with the bath of example 8and the friction test used ASTM test D27 14-68.

DESCRIPTION OF THE PREFERRED EMBODIMENTS All the alloys which wereemployed were sand cast or 5 cast in permanent molds or cast andmechanically worked.

Examples of the alloys used and the analysis of their contents are asfollows:

DESIG- ALLOY ALLOY ALLOY ALLOY NATION: Y 124 138 244 COMPO- s1 0.5 11-1317-19 23-26 SITION: Cu 3.5-4.5 0.8-1.5 0.8-1.3 1.0-1.7 Ni 1.75-2.250.8-1.3 0.8-1.3 0.8-1.3 Mg 1.25-1.75 0 8-l 3 0.8-1.3 0.5-1.0

Fe 0.6 0.7 0.7 0.7 Ti 0.2 0.2 0.2 0.2 Mn 0.2 0.2 0.2 0.2

Zn 0.2 0.2 0.2 0.2 Cr 0.3-0.5 Al -93 82-85 76-80 69-73 These alloys arewell known and are designated as indicated above or are designated byother means as shown in the following table.

The above four types of alloys were subjected to the exposure of cyanideions by immersing the alloy partially or completely into the appropriatesalt bath, for a period of time of 8 to 12 hours at a temperature of 500to 530C. After the exposure the test piece was rinsed with water and thehardness of the treated surface at several areas was determined. 1

EXAMPLES l 7 Examples of salt baths employed are as follows:

SALT BATH A 98 percent By weight of sodium cyanide and 2 percent byweight of boron oxide (B 0 SALT BATH B 90 percent By weight of sodiumcyanide, 3 percent by weight of sodium hydroxide, 1 percent by weight ofsodium carbonate and 6 percent by weight of water vapor.

The results achieved with the treated alloys after rinsing with waterare set forth below and stated in Vickers Hardness (HV) in kglmm Thehardness of untreated aluminum alloy (HV) is to kglmm SURFACE HARDENINGOF TREATED ALUMINUM ALLOYS MICROHARDNESS VlCKERS EXAMPLE 1 4 6 7 ALLOY-YALLOY-124 ALLOY-l 38 ALLOY244 BATH A BATH B BATH A BATH B BATH A BATH BBATH A BATH B HV(kg/rnm HV(kg/rr1rn HV(kg/rnm HV(kg/mm HV(kg/mm")HV(kg/mm HV(kg/mm HV(kg/mm 560 860 400 640 1200 510 400 520 760 540 3601390 760 401 540 900 600 860 l 150 260 940 320 680 950 260 500 320 l 100480 820 1000 620 535 940 1 100 l lOO 610 1400 2 2. A process accordingto claim 1, wherein sa1d aluminum alloy has the following alloymgelement con- EXAMPLE 8 Alloy 124 Alloy l 38 We claim:

1. A process for increasing the surface hardness of cast or cast andmechanically worked aluminum alloys wherein said aluminum alloy has thefollowing alloying element content:

Si 0.2 to 30.0

Cu 0.2 to 5.6

Ni 0.2 to 6.0

Mg 0.2 to 5.0

Fe 0.0 to 1.0

Ti 0.0 to 0.2

Mn 0.0 to 1.5

Zn 0.0 to 2.0

Cr 0.0 to 0.7 which comprises immersing said aluminum alloy into amolten source having a major amount of cyanide at a temperature of about430 to 550 C. for a period of time of at least about one-half hour.

tent:

Si 11 to 13 Cu 0.8 to 1.5

Ni 0.8 to 1.3

Mg 0.8 to 1.3

A1 82 to 85.

3. A process according to claim 1, wherein said aluminum alloy has thefollowing alloying element content:

Si 17 to 19 Cu 0.8 to 1.3

Ni 0.8 to 1.3

Mg 0.8 to 1.3

Al 76 to 80.

4. A process according to claim 1, which comprises using as a cyanidesource a bath of 98% sodium cyanide and 2 percent boron oxide.

5. A process according to claim 1, which comprises using as a cyanidesource a melt of percent sodium cyanide, 3 percent sodium hydroxide, 1%sodium carbonate and 6 percent water.

6. A process according to claim 1, which comprises carrying out thetreatment for a period of 8 to 12 hours.

7. A process according to claim 1, wherein the surface hardness isincreased from about 250 kg/mm to about 1400 kglmm

1. A PROCESS FOR INCREASING THE SURFACE HARDNESS OF CAST OR CAST ANDMECHANICALLY WORKED ALUMINUM ALLOYS WHEREIN SAID ALUNINUM ALLOY HAS THEFOLLOWING ALLOYING ELEMENT CONTENT: SI % 0.2 TO 30.0 CU % 0.2 TO 5.6 NI% 0.2 TO 6.0 MG % 0.2 TO 5.0 FE % 0.0 TO 1.0 TI % 0.0 TO 0.2 MN % 0.0 TO1.5 ZN % 0.0 TO 2.0 CR % 0.0 TO 0.7 WHICH COMPRISES IMMERSING ALUMINUMALLOY INTO A MOLTEN SOURCE HAVING A MAJOR AMOUNT OF CYANIDE AT ATEMPERATURE OF ABOUT 430* TO 550*C. FOR A PERIOD OF TIME OF AT LEASTABOUT ONE-HALF HOUR.
 2. A process according to claim 1, wherein saidaluminum alloy has the following alloying element content: Si % 11 to 13Cu % 0.8 to 1.5 Ni % 0.8 to 1.3 Mg % 0.8 to 1.3 Fe % < 0.7 Ti % < 0.2 Mn% < 0.2 Zn % < 0.2 Cr % A1 % 82 to
 85. 3. A process according to claim1, wherein said aluminum alloy has the following alloying elementcontent: Si % 17 to 19 Cu % 0.8 to 1.3 Ni % 0.8 to 1.3 Mg % 0.8 to 1.3Fe % < 0.7 Ti % < 0.2 Mn % < 0.2 Zn % < 0.2 Cr % A1 % 76 to
 80. 4. Aprocess according to claim 1, which comprises using as a cyanide sourcea bath of 98 percent sodium cyanide and 2 percent boron oxide.
 5. Aprocess according to claim 1, which comprises using as a cyanide sourcea melt of 90 percent sodium cyanide, 3 percent sodium hydroxide, 1percent sodium carbonate and 6 percent water.
 6. A process according toclaim 1, which comprises carrying out the treatment for a period of 8 to12 hours.
 7. A process according to claim 1, wherein the surfacehardness is increased from about 250 kg/mm2 to about 1400 kg/mm2.